1. Field of the Invention
The present invention relates to a bulb socket structure for an onboard interior lighting system provided to a ceiling of a vehicle.
2. Description of the Related Art
FIG. 1 is an exploded, perspective view showing a conventional model of bulb socket structure disclosed in Japanese Patent Application Laid-Open No. Hei. 7-254466 (Patent Document 1). FIG. 2 is a front view of a chief section of the conventional model of bulb socket structure. This conventional model of bulb socket structure is of a type to which a wedge base bulb 120 is attached. This model of bulb socket structure includes: a resin-made socket main body 100 (equivalent to a housing); and paired power supply terminals 110, 110 fixed to the inside of a bulb attachment hole 101 of the socket main body 100. This bulb socket structure enables a base 122 of the wedge base bulb 120 to be attached to the paired power supply terminals 110, 110.
Each power supply terminal 110 includes: a base plate 111 fitted into a fitting groove 102 of the socket main body 100; paired holding pieces 112, 112 folded upright from the base plate 111; and paired restricting protrusions 113, 113 folded upright from the base plate 111. The proximal end sides of the respective paired holding pieces 112, 112 are fixed to the base plate 111. The distal end sides of the paired holding pieces 112, 112 are free ends, respectively. A portion between the proximal end and the free end of one holding piece 112 and a portion between the proximal end and the free end of the other holding piece 112 are both bent inward in such a way as to come closer to each other. The restricting protrusions 113, 113 restrict excessive deformations of the holding pieces 112, 112, respectively.
The wedge base bulb (hereinafter referred to as a “bulb”) 120 includes: a bulb part 121 having a filament housed therein; and the base 122 which is flat in shape, and which is formed integrally with the bulb part 121 at the lower portion of the bulb part 121. Concave parts 123 are symmetrically formed in the base 122. When the inward-bent portions of the paired holding pieces 112, 112 of each power supply terminal 110 engage with the concave parts 123, electricity is conducted between the terminals of the base 122 and the power supply terminals 110, 110 through their contacts. In addition, a columnar part 124 is provided in the center portion of the base 122 in its width direction.
When the bulb 120 is attached to this bulb socket structure, the bulb 120 is inserted into the bulb attachment hole 101 in the socket main body 100 with the base 122 facing the bulb attachment hole 101. Once the bulb 120 is inserted in the bulb attachment hole 101, the extremity of the base 122 abuts onto the inner surfaces of the distal ends of the paired holding pieces 112, 112 of each power supply terminal 110, respectively. When the bulb 120 is inserted further into the bulb attachment hole 101 from this position, the free end sides of the paired holding pieces 112, 112 deform in directions in which the free ends of the paired holding pieces 112, 112 are separated away from each other. This deformation allows the base 122 of the bulb 120 to be inserted into the interstice between the paired holding pieces 112, 112 beyond the inward-bent portions of the paired holding pieces 112, 112. When the base 122 of the bulb 120 is inserted to a location sufficiently deep so that the concave parts 123, 123 of the base 122 reach the inward-bent portions of the holding pieces 112, 112, the holding pieces 112, 112 return to their initial positions while deforming in directions in which the holding pieces 112, 112 thereof come closer to each other. Thus, the inward-bent portion of each holding piece 112 is fitted into and engaged with its corresponding concave part 123 of the base 122. Thereby, the bulb 120 finishes being attached to the bulb socket structure, and the base 122 of the bulb 120 is held by the paired holding pieces 112, 112 of each power supply terminal 110 with their forces produced due to return from the deformation.
An onboard interior lighting system disclosed in another issue of Japanese Patent Gazette which carries Japanese Patent Application Laid-Open No. 2007-245795 (Patent Document 2) has been known as a conventional model of lighting system using a wedge base bulb as a light source. This onboard interior lighting system has a functional section which is provided on the exterior side of a ceiling board of a vehicle. A bulb socket structure to which a bulb is designed to be attached diagonally is provided to a resin-made housing constituting the main body of the functional section.
In the case of the bulb socket structures described in each patent document, it is likely that the base of the bulb may collide against the power supply terminals in the bulb attachment hole when the wedge base bulb is inserted into the bulb attachment hole, and that the power supply terminals may be accordingly deformed. In particular in the case of the bulb socket structure for the lighting system disclosed in Patent Document 2, since the bulb is designed to be attached to the bulb socket structure obliquely to the functional section main body, the power supply terminals tend to be deformed by the base if the wedge base bulb is inserted into the bulb attachment hole at a wrong angle.
In the case of each conventional model of bulb socket structure, the bulb is designed to be held by the holding pieces of each power supply terminal with their spring forces (forces produced due to return from deformation) only. For this reason, after the bulb is inserted into and detached from the holding pieces of each power supply terminal repeatedly, the spring forces of the holding pieces of each power supply terminal decrease, and the holding forces of the holding pieces thereof accordingly reduces. As a result, the bulb tends to be loose and shaky in the holding pieces thereof. Once the bulb becomes loose and shaky in the holding pieces thereof, the bulb contacts the power supply terminals 110 in a wrong condition, and the shake of the bulb causes abnormal noises. These are problems with the conventional model of bulb socket structure.